Network Working Group S. Floyd
Request for Comments: 5622 ICIR
Category: Experimental E. Kohler
UCLA
August 2009
Profile for Datagram Congestion Control Protocol (DCCP)Congestion ID 4: TCP-Friendly Rate Control for Small Packets (TFRC-SP)
Abstract
This document specifies a profile for Congestion Control Identifier
4, the small-packet variant of TCP-Friendly Rate Control (TFRC), in
the Datagram Congestion Control Protocol (DCCP). CCID 4 is for
experimental use, and uses TFRC-SP (RFC 4828), a variant of TFRC
designed for applications that send small packets. CCID 4 is
considered experimental because TFRC-SP is itself experimental, and
is not proposed for widespread deployment in the global Internet at
this time. The goal for TFRC-SP is to achieve roughly the same
bandwidth in bits per second (bps) as a TCP flow using packets of up
to 1500 bytes but experiencing the same level of congestion. CCID 4
is for use for senders that send small packets and would like a TCP-
friendly sending rate, possibly with Explicit Congestion Notification
(ECN), while minimizing abrupt rate changes.
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
Floyd & Kohler Experimental [Page 1]

RFC 5622 Profile for DCCP CCID 4 August 200914. IANA Considerations ...........................................1714.1. Reset Codes ..............................................1714.2. Option Types .............................................1714.3. Feature Numbers ..........................................1815. Thanks ........................................................1816. References ....................................................1816.1. Normative References .....................................1816.2. Informative References ...................................19
List of Tables
Table 1: DCCP CCID 4 Options .......................................9
Table 2: DCCP CCID 4 Feature Numbers ...............................91. Introduction
This document specifies an experimental profile for Congestion
Control Identifier 4, TCP-Friendly Rate Control for Small Packets
(TFRC-SP), in the Datagram Congestion Control Protocol (DCCP)
[RFC4340]. CCID 4 is a modified version of Congestion Control
Identifier 3, CCID 3, which has been specified in [RFC4342]. This
document assumes that the reader is familiar with CCID 3, instead of
repeating from that document unnecessarily.
CCID 3 uses TCP-Friendly Rate Control (TFRC), which is now specified
in RFC 5348 [RFC5348]. CCID 4 differs from CCID 3 in that CCID 4
uses TFRC-SP [RFC4828], an experimental, small-packet variant of
TFRC. The original specification of TFRC, RFC 3448 [RFC3448], has
been obsoleted by RFC 5348. The CCID 3 and TFRC-SP documents both
predate RFC 5348 and refer instead to RFC 3448 for the specification
of TFRC. However, this document assumes that RFC 5348 will be used
instead of RFC 3448 for the specification of TFRC.
CCID 4 differs from CCID 3 only in the following respects:
o Header size: For TFRC-SP, the allowed transmit rate in bytes per
second is reduced by a factor that accounts for packet header
size. This is specified for TFRC-SP in Section 4.2 of [RFC4828],
and described for CCID 4 in Section 5 below.
o Maximum sending rate: TFRC-SP enforces a minimum interval of 10
milliseconds between data packets. This is specified for TFRC-SP
in Section 4.3 of [RFC4828], and described for CCID 4 in Section 5
below.
o Loss rates for short loss intervals: For short loss intervals of
at most two round-trip times (RTTs), the loss rate is computed by
counting the actual number of packets lost or marked. For such a
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RFC 5622 Profile for DCCP CCID 4 August 2009
short loss interval with N data packets, including K lost or
marked data packets, the loss interval length is calculated as
N/K, instead of as N. This is specified for TFRC-SP in Section4.4 of [RFC4828]. If the sender is computing the loss event rate,
the Dropped Packets option specified in Section 8.7 is required,
in addition to the default CCID 3's Loss Intervals option.
Section 8.7 describes the use of the Dropped Packets option in
calculating the loss event rate. The computation of the loss rate
by the receiver for the Loss Event Rate option is described for
CCID 4 in Section 8.4 below.
o The nominal segment size: In TFRC-SP, the nominal segment size
used by the TCP throughput equation is set to 1460 bytes. This is
specified for TFRC-SP in Section 4.5 of [RFC4828], and described
for CCID 4 in Section 5 below.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Additional terminology is described in Section 2 of [RFC4342].
3. Usage
Like CCID 3, CCID 4's congestion control is appropriate for flows
that would prefer to minimize abrupt changes in the sending rate,
including streaming media applications with small or moderate
receiver buffering before playback.
CCID 4 is designed to be used either by applications that use a small
fixed segment size, or by applications that change their sending rate
by varying the segment size. If CCID 4 is used by an application
that varies its segment size in response to changes in the allowed
sending rate in bps, we note that CCID 4 doesn't dictate the segment
size to be used by the application; this is done by the application
itself. The CCID 4 sender determines the allowed sending rate in
bps, in response to on-going feedback from the CCID 4 receiver, and
the application can use information about the current allowed sending
rate to decide whether to change the current segment size.
We note that in some environments, there will be a feedback loop,
with changes in the packet size or in the sending rate in bps
affecting congestion along the path, therefore affecting the allowed
sending rate in the future.
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RFC 5622 Profile for DCCP CCID 4 August 20093.1. Relationship with TFRC and TFRC-SP
The congestion control mechanisms described here follow the TFRC-SP
mechanism specified in [RFC4828]. As with CCID 3, conformant CCID 4
implementations MAY track updates to the TCP throughput equation
directly, as updates are standardized in the IETF, rather than
waiting for revisions of this document. This document is based on
CCID 3 [RFC4342], TFRC, and TFRC-SP. For TFRC, RFC 3448 [RFC3448]
has been obsoleted by RFC 5348 [RFC5348].
3.2. Example Half-Connection
This example shows the typical progress of a half-connection using
CCID 4's TFRC Congestion Control, not including connection initiation
and termination. The example is informative, not normative. This
example differs from that for CCID 3 in [RFC4342] only in one
respect; with CCID 4, the allowed transmit rate is determined by
[RFC4828] as well as by [RFC5348].
1. The sender transmits DCCP-Data packets, where the sending rate is
governed by the allowed transmit rate as specified in [RFC4828].
Each DCCP-Data packet has a sequence number, and the DCCP header's
CCVal field contains the window counter value, used by the
receiver in determining when multiple losses belong in a single
loss event.
In the typical case of an ECN-capable half-connection, each DCCP-
Data and DCCP-DataAck packet is sent as ECN-capable, with either
the ECT(0) or the ECT(1) codepoint set. The use of the ECN Nonce
with TFRC is described in Section 9.
2. The receiver sends DCCP-Ack packets, acknowledging the data
packets at least once per round-trip time, unless the sender is
sending at a rate of less than one packet per round-trip time
[RFC5348] (Section 6). Each DCCP-Ack packet uses a sequence
number, identifying the most recent packet received from the
sender and includes feedback about the recent loss intervals
experienced by the receiver.
3. The sender continues sending DCCP-Data packets as controlled by
the allowed transmit rate. Upon receiving DCCP-Ack packets, the
sender updates its allowed transmit rate as specified in [RFC5348]
(Section 4.3) and [RFC4828]. This update is based upon a loss
event rate calculated by the sender, based on the receiver's
loss-interval feedback. If it prefers, the sender can also use a
loss event rate calculated and reported by the receiver.
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RFC 5622 Profile for DCCP CCID 4 August 2009
4. The sender estimates round-trip times and calculates a nofeedback
time, as specified in [RFC5348] (Section 4.4). If no feedback is
received from the receiver in that time (at least four round-trip
times), the sender halves its sending rate.
4. Connection Establishment
The connection establishment is as specified in Section 4 of
[RFC4342].
5. Congestion Control on Data Packets
CCID 4 uses the congestion control mechanisms of TFRC [RFC5348] and
TFRC-SP [RFC4828]. [RFC4828] MUST be considered normative except
where specifically indicated.
Loss Event Rate
As with CCID 3, the basic operation of CCID 4 centers around the
calculation of a loss event rate: the number of loss events as a
fraction of the number of packets transmitted, weighted over the last
several loss intervals. For CCID 4, this loss event rate, a round-
trip time estimate, and a nominal packet size of 1460 bytes are
plugged into the TCP throughput equation, as specified in RFC 5348
(Section 3.1) and [RFC4828].
Because CCID 4 is intended for applications that send small packets,
the allowed transmit rate derived from the TCP throughput equation is
reduced by a factor that accounts for packet header size, as
specified in Section 4.2 of [RFC4828]. The header size on data
packets is estimated as 36 bytes (20 bytes for the IPv4 header and 16
bytes for the DCCP-Data header with 48-bit sequence numbers). If the
DCCP sender is sending N-byte data packets, the allowed transmit rate
is reduced by N/(N+36). CCID 4 senders are limited to this fair
rate. The header size would be 32 bytes instead of 36 bytes when
24-bit sequence numbers were used in the DCCP-Data header.
As explained in Section 4.2 of [RFC4828], the actual header could be
larger or smaller than the assumed value due to IP or DCCP options,
IPv6, IP tunnels, header compression, and the like. Because we are
only aiming at rough fairness, and at a rough incentive for
applications, the default use of a 32-byte or 36-byte header in the
calculations of the header bandwidth is sufficient for both IPv4 and
IPv6.
If the sender is calculating the loss event rate itself, the loss
event rate can be calculated using recent loss interval lengths
reported by the receiver. Loss intervals are precisely defined in
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RFC 5622 Profile for DCCP CCID 4 August 2009Section 6.1 of [RFC4342], with the modification in [RFC4828] (Section3) for loss intervals of at most two round-trip times. In summary, a
loss interval is up to 1 RTT of possibly lost or ECN-marked data
packets, followed by an arbitrary number of non-dropped, non-marked
data packets. The CCID 3 Loss Intervals option is used to report
loss interval lengths; see Section 8.6.
For loss intervals of at most two round-trip times, CCID 4 calculates
the loss event rate for that interval by counting the number of
packets lost or marked, as described in Section 4.4 of [RFC4828].
Thus, for such a short loss interval with N data packets, including K
lost or marked data packets, the loss interval length is calculated
as N/K, instead as N. The Dropped Packets option is used to report
K, the count of lost or marked data packets.
Unlike CCID 3, the CCID 4 sender enforces a minimum interval of 10 ms
between data packets, regardless of the allowed transmit rate. If
operating system scheduling granularity makes this impractical, up to
one additional packet MAY be sent per timeslice, providing that no
more than three packets are sent in any 30 ms interval.
Other Congestion Control Mechanisms
The other congestion control mechanisms such as slow-start and
feedback packets are exactly as in CCID 3, and are described in the
subsection on "Other Congestion Control Mechanisms" of Section 5 in
[RFC4342].
5.1. Response to Idle and Application-Limited Periods
This is described in Section 5.1 of [RFC4342]. If Faster Restart is
standardized in the IETF for TFRC [KFS07], then Faster Restart MAY be
implemented in CCID4 without having to wait for an explicit update to
this document.
5.2. Response to Data Dropped and Slow Receiver
This is described in Section 5.2 of [RFC4342].
5.3. Packet Sizes
CCID 4 is intended for applications that use a fixed small segment
size, or that vary their segment size in response to congestion.
The CCID 4 sender uses a segment size of 1460 bytes in the TCP
throughput equation. This gives the CCID 4 sender roughly the same
sending rate in bytes per second as a TFRC flow using 1460-byte
segments but experiencing the same packet drop rate.
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RFC 5622 Profile for DCCP CCID 4 August 20096. Acknowledgements
The acknowledgements are as specified in Section 6 of [RFC4342] with
the exception of the Loss Interval lengths specified below.
6.1. Loss Interval Definition
The loss interval definition is as defined in Section 6.1 of
[RFC4342], except as specified below. Section 6.1.1 of RFC 4342
specifies that for all loss intervals except the first one, the data
length equals the sequence length minus the number of non-data
packets the sender transmitted during the loss interval, with a
minimum data length of one packet. For short loss intervals of at
most two round-trip times, TFRC-SP computes the loss interval length
as the data length divided by the number of dropped or marked data
packets (rather than as the data length of the loss interval).
Section 5.4 of RFC 4342 describes when to use the most recent loss
interval in the calculation of the average loss interval. [RFC4828]
adds to this procedure the restriction that the most recent loss
interval is only used in the calculation of the average loss interval
if the most recent loss interval is greater than two round-trip
times. The pseudocode is given in Section 3 of [RFC4828].
6.2. Congestion Control on Acknowledgements
The congestion control on acknowledgements is as specified in Section6.2 of [RFC4342].
6.3. Acknowledgements of Acknowledgements
Procedures for the acknowledgement of acknowledgements are as
specified in Section 6.3 of [RFC4342].
6.4. Quiescence
The procedure for detecting that the sender has gone quiescent is as
specified in Section 6.4 of [RFC4342].
7. Explicit Congestion Notification
Procedures for the use of Explicit Congestion Notification (ECN) are
as specified in Section 7 of [RFC4342].
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RFC 5622 Profile for DCCP CCID 4 August 20098. Options and Features
CCID 4 can make use of DCCP's Ack Vector, Timestamp, Timestamp Echo,
and Elapsed Time options, and its Send Ack Vector and ECN Incapable
features. CCID 4 also imports the currently defined CCID-3-specific
options and features [RFC4342], augmented by the Dropped Packets
option specified in this document. Each CCID4-specific option and
feature contains the same data as the corresponding CCID 3 option or
feature, and is interpreted in the same way, except as specified
elsewhere in this document (or in a subsequent IETF standards-track
RFC that updates or obsoletes this specification).
Option DCCP- Section
Type Length Meaning Data? Reference
----- ------ ------- ----- ---------
128-183 Unassigned
184-190 Reserved for
experimental
and testing use
191 Unassigned
192 6 Loss Event Rate N 8.5
193 variable Loss Intervals N 8.6
194 6 Receive Rate N 8.3
195 variable Dropped Packets N 8.7
196-247 Unassigned
248-254 Reserved for
experimental
and testing use
255 Unassigned
Table 1: DCCP CCID 4 Options
The "DCCP-Data?" column indicates that all currently defined CCID4-
specific options MUST be ignored when they occur on DCCP-Data
packets.
As with CCID 3, the following CCID-specific features are also
defined.
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RFC 5622 Profile for DCCP CCID 4 August 2009
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
128-183 Unassigned
184-190 Reserved for experimental
and testing use
191 Unassigned
192 Send Loss Event Rate SP 0 N 8.4
193-247 Unassigned
248-254 Reserved for experimental
and testing use
255 Unassigned
Table 2: DCCP CCID 4 Feature Numbers
More information is available in Section 8 of [RFC4342].
8.1. Window Counter Value
The use of the Window Counter Value in the DCCP generic header's
CCVal field is as specified in Section 8.1 of [RFC4342]. In addition
to their use described in CCID 3, the CCVal counters are used by the
receiver in CCID 4 to determine when the length of a loss interval is
at most two round-trip times. None of these procedures require the
receiver to maintain an explicit estimate of the round-trip time.
However, Section 8.1 of [RFC4342] gives a procedure that implementors
may use if they wish to keep such an RTT estimate using CCVal.
8.2. Elapsed Time Options
The use of the Elapsed Time option is defined in Section 8.2 of
[RFC4342].
8.3. Receive Rate Option
The Receive Rate option is as specified in Section 8.3 of [RFC4342].
8.4. Send Loss Event Rate Feature
The Send Loss Event Rate feature is as defined in Section 8.4 of
[RFC4342].
See [RFC5348], Section 5, and [RFC4828], Section 4.4, for a normative
calculation of the loss event rate. Section 4.4 of [RFC4828]
modifies the calculation of the loss interval size for loss intervals
of at most two round-trip times.
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RFC 5622 Profile for DCCP CCID 4 August 2009
If the CCID 4 receiver is using the Loss Event Rate option, the
receiver needs to be able to determine if a loss interval is short,
of at most two round-trip times. The receiver can heuristically
detect a short loss interval by using the Window Counter in arriving
data packets. The sender increases the Window Counter by 1 every
quarter of a round-trip time, with the caveat that the Window Counter
is never increased by more than five, modulo 16, from one data packet
to the next. Using the Window Counter to detect loss intervals of at
most two round-trip times could result in some false positives, with
some longer loss intervals incorrectly identified as short ones. For
example, if the loss interval contained data packets with only two
Window Counter values, say, k and k+5, then the receiver could not
tell if the loss interval was at most two round-trip times long or
not. Similarly, if the sender sent data packets with Window Counter
values of 4, 8, 12, 0, 5, but the packets with Window Counter values
of 8, 12, and 0 were lost in the network, then the receiver would
only receive data packets with Window Counter values of 4 and 5, and
would incorrectly infer that the loss interval was at most two round-
trip times.
8.5. Loss Event Rate Option
The Loss Event Rate option is as specified in Section 8.5 of
[RFC4342].
See [RFC5348] (Section 5) and [RFC4828] for a normative calculation
of the loss event rate.
8.6. Loss Intervals Option
The Loss Intervals option is as specified in Section 8.6 of
[RFC4342].
8.7. Dropped Packets Option
This section describes the Dropped Packets option, a mechanism for
reporting the number of lost and marked packets per loss interval.
By reporting both the Loss Intervals and Dropped Packets options on
the feedback packets, the receiver gives the sender sufficient
information to calculate the loss event rate, or to verify the
calculation of the reported loss event rate, if the sender so
desires.
The core information reported by CCID 4 receivers is a list of recent
loss intervals, where a loss interval begins with a lost or ECN-
marked data packet; continues with at most one round-trip time's
worth of packets that may or may not be lost or marked; and completes
with an arbitrarily long series of non-dropped, non-marked data
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RFC 5622 Profile for DCCP CCID 4 August 2009
packets. Loss intervals model the congestion behavior of TCP NewReno
senders, which reduce their sending rate at most once per window of
data packets. Consequently, the number of packets lost in a loss
interval is not important for either TCP's or TFRC's congestion
response. CCID 3's Loss Intervals option reports the length of each
loss interval's lossy part, not the number of packets that were
actually lost or marked in that lossy part.
However, for computing the loss event rate for periods that include
short loss intervals the TFRC-SP sender needs to know the number of
packets lost or marked in a loss interval, over and above the length
of the loss interval in packets. The Dropped Packets option, a
CCID4-specific option, reports this information. Together with the
existing Loss Intervals option, the Dropped Packets option allows the
CCID 4 sender to discover exactly how many packets were dropped from
each loss interval. The receiver reports the number of lost or
marked packets in its recently observed loss intervals using the
Dropped Packets option.
The Dropped Packets Option is specified as follows:
+--------+--------+-------...-------+--------+-------
|11000011| Length | Drop Count | More Drop Counts...
+--------+--------+-------...-------+--------+-------
Type=195 3 bytes
Figure 1: Dropped Packets Option
The Dropped Packets option contains information about one to 84
consecutive loss intervals, always including the most recent loss
interval. As with the Loss Intervals option, intervals are listed in
reverse chronological order. Should more than 84 loss intervals need
to be reported, multiple Dropped Packets options can be sent; the
second option begins where the first left off, and so forth.
One Drop Count is specified per loss interval. Drop Count is a 24-
bit number that equals the number of packets, lost or received, ECN-
marked during the corresponding loss interval. By definition, this
number MUST NOT exceed the corresponding loss interval's Loss Length.
CCID 4 receivers MUST report Dropped Packets options with every
feedback packet. Any packet containing a Loss Intervals option MUST
also contain a Dropped Packets option covering the same loss
intervals. If a feedback packet does not include a relevant Dropped
Packets option, and the CCID 4 sender is computing the loss event
rate itself, the sender MUST treat the relevant loss intervals' Drop
Counts as equal to the corresponding Loss Lengths, as specified
below.
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RFC 5622 Profile for DCCP CCID 4 August 2009
Consider a CCID 4 receiver. As specified in Section 8.6.1 of RFC4342, the receiver sends the Loss Intervals option for all intervals
that have not been acknowledged by the sender. When this receiver
sends a feedback packet containing information about the N most
recent loss intervals (packaged in one or more Loss Intervals
options), then the receiver includes on the same feedback packet one
or more Dropped Packets options covering exactly those N loss
intervals. CCID 4 senders MUST ignore Drop Counts information for
loss intervals not covered by a Loss Intervals option on the same
feedback packet. Conversely, a CCID 4 sender might want to
interpolate Drop Counts information for a loss interval not covered
by any Dropped Packets options; such a sender MUST use the
corresponding loss interval's Loss Length as its Drop Count.
Each loss interval's Drop Count MUST, by definition, be less than or
equal to its Loss Length. A Drop Count that exceeds the
corresponding Loss Length MUST be treated as equal to the Loss
Length.
8.7.1. Example
Consider the following sequence of packets, where "-" represents a
safely delivered packet and "*" represents a lost or marked packet.
This sequence is repeated from [RFC4342].
Sequence
Numbers: 0 10 20 30 40 44
| | | | | |
----------*--------***-*--------*----------*-
Figure 2: Sequence of Delivered (-) and Lost (*) Packets
Assuming that packet 43 was lost, not marked, this sequence might be
divided into loss intervals as follows:
0 10 20 30 40 44
| | | | | |
----------*--------***-*--------*----------*-
\________/\_______/\___________/\_________/
L0 L1 L2 L3
Figure 3: Loss Intervals for the Packet Sequence
A Loss Intervals option sent on a packet with Acknowledgement Number
44 to acknowledge this set of loss intervals might contain the bytes
193,39,2, 0,0,10, 128,0,1, 0,0,10, 0,0,8, 0,0,5, 0,0,10, 0,0,8,
0,0,1, 0,0,8, 0,0,10, 128,0,0, 0,0,15; for interpretation of this
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RFC 5622 Profile for DCCP CCID 4 August 2009
option, see [RFC4342]. A Dropped Packets option sent in tandem on
this packet would contain the bytes 195,14, 0,0,1, 0,0,4, 0,0,1,
0,0,0. This is interpreted as follows.
195 The Dropped Packets option number.
14 The length of the option, including option type and length
bytes. This option contains information about (14 - 2)/3 =
4 loss intervals. Note that the two most recent sequence
numbers are not yet part of any loss interval -- the Loss
Intervals option includes them in its Skip Length -- and are
thus not included in the Dropped Packets option.
0,0,1 These bytes define the Drop Count for L3, which is 1. As
required, the Drop Count is less than or equal to L3's Loss
Length, which is also 1.
0,0,4 The Drop Count for L2 is 4.
0,0,1 The Drop Count for L1 is 1.
0,0,0 Finally, the Drop Count for L0 is 0.
9. Verifying Congestion Control Compliance with ECN
Verifying congestion control compliance with ECN is as discussed in
Section 9 of [RFC4342].
9.1. Verifying the ECN Nonce Echo
Procedures for verifying the ECN Nonce Echo are as specified in
Section 9.1 of [RFC4342].
9.2. Verifying the Reported Loss Intervals and Loss Event RateSection 9.2 of [RFC4342] discusses the sender's possible verification
of loss intervals and loss event rate information reported by the
receiver.
10. Implementation Issues10.1. Timestamp Usage
The use of the Timestamp option is as discussed in Section 10.1 of
[RFC4342].
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RFC 5622 Profile for DCCP CCID 4 August 200910.2. Determining Loss Events at the Receiver
The use of the window counter by the receiver to determine if
multiple lost packets belong to the same loss event is as described
in Section 10.2 of [RFC4342].
10.3. Sending Feedback Packets
The procedure for sending feedback packets is as described in Section10.3 of [RFC4342].
11. Design Considerations
This section discusses design considerations for the field sizes in
the Loss Intervals and Dropped Packets options.
11.1. The Field Size in the Loss Intervals OptionSection 8.6 of RFC 4342 specifies a Loss Intervals option with three
fields for each loss interval, for reporting the Lossless Length,
Loss Length, and Data Length. Each field is specified to be three
bytes. Section 8.6 of this document specifies that CCID 4 use the
same Loss Intervals option as CCID 3, with the same field sizes.
This has the significant advantage of minimizing the implementation
differences between CCID 3 and CCID 4. However, it has been
suggested that CCID 4 *could* use a Loss Intervals option with
smaller field sizes, since a CCID 4 sender enforces a minimum
interval of 10 ms between data packets. This section explains the
reason for CCID 4 to use the same Loss Intervals option as specified
for CCID 3.
The Lossless Length field reports the number of packets in the loss
intervals' lossless part, and the Loss Length field reports the
number of packets in the loss interval's lossy part. The Data Length
field reports the number of packets in the loss interval's data
length (excluding non-data packets). A two-byte Data Length field
can report a data length of 65,536 packets, corresponding to a loss
event rate of 0.00002; this is enough to give the CCID 4 sender an
allowed sending rate of roughly 250 packets per RTT, which is enough
for a connection with a round-trip time of at most 2.5 seconds. For
a CCID 4 connection with a larger round-trip time, the three-byte
Lossless Length and Data Length fields would be needed.
For the Loss Length field in the Loss Intervals option, reporting the
number of packets in the one-RTT lossy part of the loss interval, a
one-byte field would not be sufficient for a CCID 4 connection with a
long RTT (three seconds or longer). For the Loss Length field, a
two-byte field should be sufficient for CCID 4. However, our
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RFC 5622 Profile for DCCP CCID 4 August 2009
judgement is that the advantages of using the same Loss Intervals
option as in CCID 3 outweigh any advantages of using a CCID 4 Loss
Intervals option that uses eight bytes instead of nine bytes for
reporting the fields for each loss interval.
11.2. The Field Size in the Dropped Packets OptionSection 8.7 specifies the Dropped Packets option for reporting the
number of lost or marked packets per loss interval, allocating three
bytes for the drop count field for each loss interval reported. The
three-byte field is partly for simplicity, to give the same field
size as the fields in the Loss Intervals option specified in RFC4342. It has been suggested that CCID 4 *could* use a smaller field
size for the Dropped Packets option. This section discusses the
issue of the size of the drop count field in the Dropped Packets
option.
It is not necessary to specify a three-byte field for the Dropped
Packets option. A one-byte field would allow a reported drop count
of 255, and a two-byte field would allow a reported drop count of
65,535. A two-byte field would clearly be sufficient for the drop
count field for CCID 4.
In fact, a one-byte field would *probably* be adequate for reporting
the drop count for a loss interval in a CCID 4 connection. Because a
CCID 4 sender enforces a minimum interval of 10 ms between data
packets, a sender would need a round-trip time of over 2.55 seconds
to have more than 255 packets lost or marked in a single loss
interval; round-trip times of greater than three seconds are not
unusual for some flows traversing satellite links. The drop count
field is used in CCID 4 to compute the actual loss rate for short
loss intervals, rather than using the loss event rate that is used
for longer loss intervals. If a loss interval of at most two round-
trip times included N packets sent, with more than 255 of those
packets lost or marked, a drop count field of one byte would allow a
drop count of at most 255 to be reported, resulting in a computed
loss rate for that interval of 255/N. This loss rate might be less
than the actual loss rate, but it is significantly higher than the
loss event rate of 1/N, and should be sufficient to prevent a steady-
state condition of a CCID 4 connection with multiple packets dropped
each round-trip time. Thus, a one-byte field would probably be
adequate for reporting the drop count for a loss interval in a CCID 4
connection. However, at the moment this document specifies a three-
byte field, for consistency with the field size in the Loss Intervals
option.
Floyd & Kohler Experimental [Page 16]

RFC 5622 Profile for DCCP CCID 4 August 200912. Experimental Status of This Document
TFRC-SP is a congestion control mechanism defined in RFC 4828.
Section 10 of [RFC4828] describes why TFRC-SP is currently specified
as experimental and why it is not intended for widespread deployment
at this time in the global Internet. Since TFRC-SP is Experimental,
CCID 4 is therefore also considered experimental. If the IETF
publishes a Standards-Track RFC that changes the status of TFRC-SP,
then CCID 4 should then be updated to reflect the change of status.
13. Security Considerations
Security considerations include those discussed in Section 11 of
[RFC4342]. There are no new security considerations introduced by
CCID 4.
14. IANA Considerations
This specification defines the value 4 in the DCCP CCID namespace
managed by IANA. This is a permanent codepoint, as is needed for
experimentation across the Internet using different codebases.
CCID 4 also uses three sets of numbers whose values have been
allocated by IANA, namely CCID4-specific Reset Codes, option types,
and feature numbers. This document makes no particular allocations
from the Reset Code range, except for experimental and testing use
[RFC3692]. We refer to the Standards Action policy outlined in
[RFC5226].
14.1. Reset Codes
Each entry in the DCCP CCID 4 Reset Code registry contains a CCID4-
specific Reset Code, which is a number in the range 128-255; a short
description of the Reset Code; and a reference to the RFC defining
the Reset Code. Reset Codes 184-190 and 248-254 are permanently
reserved for experimental and testing use. The remaining Reset Codes
-- 128-183, 191-247, and 255 -- are currently reserved, and should be
allocated with the Standards Action policy, which requires IESG
review and approval and Standards-Track IETF RFC publication.
14.2. Option Types
Each entry in the DCCP CCID 4 option type registry contains a CCID4-
specific option type, which is a number in the range 128-255; the
name of the option, such as "Loss Intervals"; and a reference to the
RFC defining the option type. The registry is initially populated
using the values in Table 1, in Section 8. This includes the value
195 allocated for the Dropped Packets option. This document
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